You may have seen this post from out TechTalk blog that discusses how to measure the phase angles you now know how to draw. We added it here to make sure everyone gets a chance to read this important information.

I’ve noticed that many students don’t seem to have experience with phase angle meters, and they can’t guess what a phase angle meter would measure in simple circuit. This knowledge is CRITICAL if you want to perform a protective relay meter test properly. Anyone can inject voltages and currents into a relay and record the results on a test sheet. A good relay tester can interpret those results and find MAJOR relay setting and installation problems with a simple test.

Let’s take a look at a simple DC circuit to start:

You should be able to see in our DC circuit that:

1. the voltage sets the potential energy as well as the direction of current flow,
2. the load (resistor) determines how much current will flow for a given voltage, and
3. the measured polarity is dependent on the lead connections, not the circuit.

Let’s try a more complicated circuit:

Can you fill in this chart?

Now let’s look at DC current

Can you fill in this chart?

Now let’s look at a simple AC Circuit with a phase angle meter.

Phase angle meters measure magnitudes AND angles based on a reference angle.

You can walk up to any part of a DC circuit and measure the voltage drop across (or current flow through) a device because there is only one direction in a DC system, using the metering definitions of positive to negative. (I know that physics defines it as negative to positive, but that’s not how our meters work.)

You can measure the magnitude of current and voltage in an AC system much like a DC circuit, but the AC waveform switches constantly from positive to negative with every cycle. This makes the “polarity” of the circuit much harder to define.

Our AC circuit has a starting point, but our phase angle meter has no way of recording that without a reference. We can measure the phase angle by adding a reference connection as shown below.

It is important to note that the CIRCUIT has not changed. The reason the last two angle measurements are different is caused by our CONNECTIONS, just like switching the leads on a DC meter.

Now we can put a CT in the circuit to measure the current flowing through the resistor.

100% resistive circuits will always be at 0° or 180°.

Can you fill in this chart with voltage measurements with a phase angle meter?

Can you fill in this chart with current measurements with a phase angle meter?

Imagine the CT polarity is connected to the [+] terminal as shown in the AB example below and the [+] and [Amps] designations in the chart determine the direction of the CT.

What happens if we connect our phase angle meter to non-resistive circuits?

You should be able to see by now how important the REFERENCE of your phase angle meter is. Remember, every device that measures phase angles MUST have a reference, and that reference may not be what you think it is. Some phase angle meters use whatever they are plugged into as the reference. Some use whatever is connected to V1, Van, or Vab. Some use whatever is connected to I1 or Ia. If you don’t know the reference, you can’t verify that your measurements are correct.

Also remember that the system determines the voltage magnitude and angle and the load determines the current and current angle. There is no such thing as a purely resistive, capacitive, or inductive loads. This means that you should never apply current at 0°, 90° lag, or 90° lead because your relay or meter will never see these values in the real world. Always try to simulate realistic conditions whenever possible.

You now have the foundation necessary to understand what happens during a meter test. Try to figure out what you are supposed to measure before you perform your next meter test. You can get some help with our Finding the Direction in Directional Overcurrent Relays post before we cover meter testing in a future post.